In order to manufacture a seamless steel pipe by applying a Mannesman Pipe Manufacturing Method, for example, at first a round steel piece heated up to a predetermined temperature (hereinafter called as a billet) is punched with the aforesaid plug by applying a slant type drilling machine called as a piercing machine and rolled to make a hollow raw pipe (hereinafter called as a hollow member). Then, the hollow member is rolled under application of a plug or a mandrel bar (hereinafter merely called as a bar) by the extending and rolling machine such as an elongating machine, a plug mill, a reel machine or a mandrel mill or the like in the same manner as that described above so as to reduce its wall thickness. Further, as required, after the rolled pipe is heated again, its outer diameter is reduced without using any plug in a squeezing rolling machine or a non-proportional rolling machine such as a reducing machine and a sizing machine. In this case, the hollow raw pipe rolled by a plug mill, a reel machine and a mandrel mill or the like is called as a shell.
However, the plug is always exposed under a high temperature and a high load through a continuous contact with the heated billet or hollow member at the stage of punching and rolling operation performed by the aforesaid piercing machine, resulting in that the plug is quite easily worn out and melt lost. In the prior art, since the plug was made of steel, the plug was heat treated at about 900 to 1000.degree. C. as a countermeasure against a worn-out loss and a scale film of several 10 to several 100 .mu.m was formed. However, when such a plug as above is used in making a pipe of high alloy steel containing Cr of 5 wt. % of which demand is increased in particular in recent years, the plug is merely endurable against several number of billets and its life is remarkably short.
Then, several number of technologies improving the life of the plug have been proposed by changing material quality of the plug. For example, either an official gazette of Japanese Patent Laid-Open No. Sho 60-159156 or an official gazette of Japanese Patent Laid-Open No. Sho 60-208458 proposes the plug raw material having Mo or W added to 3 wt. % Cr--1 wt. % Ni steel. However, there was a certain limitation in improvement of a life of the plug of piercing machine only through a utilization of such a material quality of steel system. For example, even if a billet with a diameter of 110 mm and a length of 2.5 m of SUS304 of austenite stainless steel is punched and rolled with a plug of the aforesaid raw material, a life of the plug is merely 3 pieces/unit (the number of billets which can be punched and rolled with one plug is three).
In addition, either an official gazette of Japanese Patent Laid-Open No. Sho 63-203205 or an official gazette of Japanese Patent Publication No. Hei 5-85242 has a proposal that a Mo group alloy is connected to the extremity end of the plug to increase a heat-resistant characteristic and an anti-wear characteristic. In addition, the official gazette of Japanese Patent Laid-Open No. Sho 62-244505 has a proposal of the same gist saying that a super-hard material is connected to the extremity end of the plug and ceramics is molten injected against the surface of the plug. In addition, the official gazette of Japanese Patent Laid-Open No. Sho 62-238011 has a proposal that a core material of the plug is made of ceramics and a metallic powder layer is hot pressed in isotropic pressure (normally called as an HIP processing). However, the plug having such a structure as above shows a disadvantage that a coupling of the raw materials is not kept during rolling operation and its life becomes shorter than that of the prior art.
In addition, the official gazette of Japanese Patent Laid-Open No. Hei 2-156037 has a proposal that there is provided a plug having a superior heat-resistant characteristic composed of a sintered member having a hard phase (a borite thermet) and a coupling phase having mainly Ni, Mo. When a billet composed of 13 wt. % Cr steel, for example, was punched under application of this plug, twelve billets could be punched with one plug. However, this degree of improvement is not sufficient for the operation.
In addition, the official gazette of Japanese Patent Laid-Open No. Sho 60-137511 has a proposal about a plug for manufacturing a seamless steel pipe in which the extremity end of ceramics is fixed to the steel substrate of the plug. Usually, the plug shows a different action against a pressed member in response to the specified location such as the extremity end, the work section and the reeling section or the like to be illustrated later. For example, the punching and rolling operation at the piercing machine is carried out such that the billet is punched at the extremity end, the produced raw pipe is rapidly decreased in its wall thickness at the work section and then the wall thickness of the raw pipe is adjusted at the reeling section. However, as described in the official gazette of Japanese Patent Laid-Open No. Sho 60-137511, even if only the extremity end is made of ceramics, it is frequently found that the work section or the reeling section is damaged, and in particular, the work section is remarkably damaged. That is, even if such a plug as above is employed for a punching and rolling operation, its life is not so extended as expected and a shape of the inner surface of the hollow member attained by the damage of the work section is not improved.
In turn, as a countermeasure for extending the life of the plug, there is provided a usage of plug in addition to a modification of the aforesaid material quality of the plug. Either the official gazette of Japanese Patent Laid-Open No. Sho 51-133167 or the official gazette of Japanese Patent Laid-Open No. Hei 1-180712, for example, has a proposal about a technology that the punching and rolling can be carried out while lubricant is being injected from the extremity end of the plug and in turn the official gazette of Japanese Patent Laid-Open No. Hei 5-138213 has a proposal about a technology that the punching and rolling can be performed after lubricant is coated in advance at the surface of the plug, respectively. However, the technology for injecting the lubricant from the extremity end of the plug shows a problem that the injection hole at the extremity end is clogged and in turn the technology for coating lubricant before punching and rolling operation shows a problem that only a required amount of lubricant can be uniformly coated at the surface of the plug. In addition, these both technologies have a common economical problem of increasing cost in manufacturing a pipe under the application of lubricant and so they may not employed in view of their industrial application.
The prior art described above related to the plug of the piercing machine in which the billet was punched and rolled. To the contrary, as for the plugs used in the slant type rolling machines such as an elongating machine and a reeling machine, various kinds of improvement technologies concerning the raw material and the method of use of the plug have been proposed in the same manner as that of the plug in the piercing machine. However, these proposed technologies (their description will be eliminated) do not accomplish the life of such a plug as one in which a pipe manufacturing company may not satisfy. In addition, as for the plug of the plug milling machine and the mandrel bar or the like to be used in the rolling of the inner surface of the hollow member, a life of the plug or the bar in a punching type rolling machine (such as a plug mill, a mandrel mill or the like) which may be sufficiently adapted can not be attained in the same manner as that described above.